Adsorption at the Solid-Liquid Interface: An Atomic Force Microscopy Study

Abstract: The aim of the work presented in this thesis was to investigate nature of adsorption of amphiphiles to the solid–liquid interface. The surface aggregate structures of the zwitterionic surfactant dodecyldimethylammoniopropanesulfonate, (DDAPS) and a series of nonionic polyoxyethlylene glycol alkyl ethers (C10E5, C12E5, C12E8, C10E6, C14E6 and C16E6) have been determined by Atomic Force Microscopy on solids of different hydrophobicity. The hydrophilic surfaces include silicon nitride, mica and silica and the hydrophobic surface are graphite and hydrophobised silica. The nonionic surfactant C12E8 has also been investigated adsorbed to a number of gold–alkanethiolate surfaces where the hydrophobicity has been systematically varied by altering the relative amounts of thiohexadecane and thiohexadecanol. In virtually every case the surface aggregates on the more hydrophilic solids were found to have a spherical or globular shape while on the more hydrophobic solids aggregation resulted in monolayer (hydrophobised silica and hydrophobic gold–alkanethiolate surfaces) or hemicylindrical aggregate (graphite) formation. The adsorbed layer geometry changes in response to the higher solid–liquid interfacial energy of the hydrophobic solids. The formation of hemicylindrical aggregates has been found to have a chain length dependence for polyoxyethlene surfactants which has been ascribed to an epitaxial ordering effect induced by the graphite lattice. Ordered, templated, thin films of the polymer ethyl(hydroxyethyl)cellulose have also been prepared on a graphite surface by exploiting the nature of sodium dodecyl sulphate adsorption and its interaction with EHEC. The orientation of the polymer networks were predominately aligned in one of three preferential directions separated by 60', presumably as a result of templating by coadsorbed SDS. In addition the normal and lateral force interactions between mono- and bilayers of L-a-dioleoyl lecithin coadsorbed at the solid–aqueous interface with n-dodecyl-b-maltoside surfactant have been investigated. The surface supported mono- and bilayer structures with controlled lipid densities were adsorbed by varying the substrate surface chemistry and the bulk concentrations of lipid and surfactant during the lipid deposition process. Strong correlations have been found between the force required to remove the adsorbed layer and the measured friction force. Triblock copolymers of the type poly(ethylene oxide-tetrahydrofuran-ethylene oxide) have been adsorbed to the silica–water interface and studied by atomic force microscopy and time resolved spectroscopy. The presence of surface micelles was indicated by both techniques.